I've always believed that without 9/11 the Housing Bubble would grow at a slower pace. The Fed cut interest rates sharply in response to the spike in monthly job losses that occurred right after the attack. So take away the attack and the early 2000's recession wouldn't have been as severe, so the Fed wouldn't have been as aggressive, which means the rates of borrowing for mortgages is lower than OTL, which means the bubble grows at a slower pace.
Now the Great Recession officially started in late 2007, and the US was losing jobs all through 2008, and there were a series of crisis during the year. Bear Stearns early in the year, the spike in oil and gas prices during the spring/summer, Fannie/Freddie in late summer, then Lehman and AIG in September. If the Housing bubble grows at a slower pace then we might be able to push back some or maybe even all of these events until after the election.
You could have a scenario where all the events happen after the 2008 election and the President who wins that year gets the mess dumped in their lap and their party loses huge in 2010 and 2012. You could get a scenario where some of the events happen in the second half of 2008 and cause the party of the sitting President to lose that November, but then the really big stuff goes off just after the new President gets sworn in. If people were pissed with Obama in 2010 because things were still messed up, even though all the big crisis occurred during the Bush Presidency, then how would the public react to a President where perhaps the economy started going bad under his Predecessor, but the big bankruptcies and the bailouts happened on his/her watch?
So you have to factor in how the absence of 9/11 alters the trajectory of the economy and how that changes the political situation.